[Refactoring] Don't crash when converting a function to async that contains a call to init

We were trying to retrieve the name of all function calls in the body using `getBaseIdentifier`. But calls to `init` don’t have a base identifier, just a `DeclBaseName` (which is special). Work with the `DeclBaseName` internally to prevent the crash.

Fixes rdar://78024731 [SR-14637]

test/refactoring/ConvertAsync/basic.swift

@@ -844,3 +844,15 @@ func testPreserveComments3() {
 // PRESERVE-TRAILING-COMMENT-CALL:      let s = await simple()
 // PRESERVE-TRAILING-COMMENT-CALL-NEXT: print(s)
 // PRESERVE-TRAILING-COMMENT-CALL-NOT:  // make sure we pickup this trailing comment if we're converting the function, but not the call
+
+class TestConvertFunctionWithCallToFunctionsWithSpecialName {
+  required init() {}
+  subscript(index: Int) -> Int { return index }
+
+// RUN: %refactor -convert-to-async -dump-text -source-filename %s -pos=%(line+1):3
+  static func example() -> Self {
+    let x = self.init()
+    _ = x[1]
+    return x
+  }
+}